1. Field of the Invention
The present invention relates to a power transmission mechanism having at least one cam-incorporated gear, and in particular, relates to a type of power transmission mechanism having at least one cam-incorporated gear and a planetary gear mechanism used to transmit rotation to the cam-incorporated gear. The present invention also relates to a power transmission mechanism of an imaging apparatus.
2. Description of the Related Art
In addition to a conventional object viewing function (optical view mode) using an optical viewfinder, SLR digital cameras having a ‘ live view mode’, in which an electronic object image captured by an image sensor is viewed in real-time on a display monitor such as an LCD before photographing, are known in the art. In the live view mode, the quick-return mirror (hereinafter referred simply to as the mirror) is held in the retracted position (mirror-up position) in which the quick-return mirror is retracted from a photographing optical path when the object is viewed before photographing, which is different from a viewing state in the optical view mode, in which the object is viewed through an optical viewfinder before photographing. Therefore, when a photographing operation is performed in the live view mode, the mirror drive operation and the shutter drive operation need to be controlled in a different manner from those in optical view mode (normal exposure mode).
In the SLR digital camera disclosed in Non-Patent Document 1 (pages 202 and 203 in “Asahi Camera” (a Japanese monthly photographic magazine published by Asahi Shimbun Publications Inc., print edition, August 2008), another image sensor (live-view image sensor) used exclusively for capturing the object image in the live view mode, independently of an image sensor used for taking pictures, is installed in a viewfinder optical system so that a quasi-live view operation can be performed using the live-view image sensor in the viewfinder optical system. In this type of camera, an electronic image captured by an image sensor different from the image sensor used at a time of exposure is viewed on a display monitor before shooting, and accordingly, this view mode cannot be considered to be live view mode in the strict sense, as there is a possibility of the image displayed on the monitor not precisely coinciding with the image taken at a time of exposure. Additionally, installation space for an image sensor needs to be secured in the viewfinder, which becomes an obstacle for miniaturization of the camera and causes an increase in production cost.
In the SLR digital camera disclosed in Non-Patent Document 2 (pages 230 and 231 in “Asahi Camera” (a Japanese monthly photographic magazine published by Asahi Shimbun Publications Inc., print edition, January 2008), the mirror drive mechanism and the shutter drive mechanism are provided with two independent drive sources, respectively, and a shutter charge operation is performed by the shutter drive mechanism with a mirror-up state being maintained by the mirror drive mechanism in a photographing operation in the live view mode. In this type of camera, a mirror drive system and a shutter drive system are each provided with an independent drive source and an independent drive mechanism, which makes it difficult to achieve miniaturization and weight reduction of the camera and causes an increase in production cost.
The applicant of the present invention has found that miniaturization, weight reduction and cost reduction of the camera body can be achieved in an SLR camera having a live view mode by solving the above described problems that reside in Non-Patent Documents 1 and 2 by employing a planetary gear mechanism as a mechanism for transmitting driving force of a motor to a mirror drive system (mechanism) and a shutter charge system (mechanism). Specifically, the intention is to carry out a photographing operation (sequence of processes for taking a picture) in the live view mode by a drive mechanism equipped with two cam-incorporated gears like the drive mechanism shown in Japanese Patent No. 3,153,482 and to transmit a driving force selectively to the two cam-incorporated gears via a planetary gear mechanism.
In the case where a power transmission mechanism of a mirror drive system or a shutter charge system in an SLR camera includes a cam-incorporated gear as a component of the power transmission mechanism, the rotational position of the cam-incorporated gear must be precisely controlled to carry out a photographing operation. For instance, in a type of power transmission mechanism as shown in Non-Patent Document 2 in which the mirror drive mechanism and the shutter drive mechanism are provided with two independent drive sources, respectively, the cam-incorporated gear can be held at a fixed rotational position by the load of a drive source in a state where the rotational driving of the cam-incorporated gear by the drive source is at rest. On the other hand, in another type of power transmission mechanism in which the driving state of the cam-incorporated gear is selected by a drive switcher such as the above described planetary gear mechanism, there is a possibility of the rotational position of the cam-incorporated gear being displaced in a state where the cam-incorporated gear is disengaged from a planet gear. If such a displacement of the rotational position occurs, a mechanical part of the camera comes to a state different from the original state, which may exert an adverse influence on the drive control of the camera. A power transmission mechanism using a cam-incorporated gear in an SLR camera has been discussed above by way of example; however, in a typical power transmission mechanism having a rotational drive member, the operation of which needs to be controlled at high precision, a similar problem arises when power other than the power of a drive source acts on the rotational drive member.